Can't Connect to R710

I've problem for my R710, I used it from april 2016, and no promblem until jun 16th user can't joint to AP, I try restart the AP and back to normal. But the problem back again every 2 or 3 days.anyone know how to fix the promblem? my AP use software version 100.2.0.0.336. and have 4ssid 5ghz and 2ssid for 2.4ghz.